My AC10 is producing either a distortion buzz or odd harmonic when playing the 1st and 2nd strings from the 11 fret upwards - when both strings are picked.If one is sounding and the other picked the noise is present. Individually the notes are clean.Its been like this from New - though it was an issue with the G3 - but now I have a new guitar its the same with both guitars plugged directly into the amp.

Yesterday I replaced the EL84's - one failed with horrific microphony. For some time I was thinking that I would change the valves and this issue would resolve.I have not changed the ECC83's

So if anyone else has an AC10 C1 - could you check if picking the 1st and 2nd strings 11th fret upwards - one after the other or together - you have this effect.